In a hot runner injection molding apparatus, melt is transferred from a melt source through a system of runners, through a nozzle and into a mold cavity through a gate. When the mold cavity is opened for the ejection of the molded part, it is important to prevent leakage of hot melt from the nozzle through the gate.
A valve pin is commonly used to seal the gate into the mold cavity prior to opening the mold cavity. The valve pin is positioned in the nozzle, and is movable into and out of the gate to control the flow of melt therethrough.
Proper alignment of the valve pin with respect to the gate can impact on the longevity of both the valve pin and the gate, and the effectiveness of the seal obtained by the valve pin and the gate. To this end, some nozzles include some form of alignment means to align the valve pin prior to its entry into the gate. However, in order for the nozzle's alignment means to be effective, the nozzle itself must be positioned in alignment with respect to the gate. Several factors generally cause the nozzle to be misaligned with respect to the gate. One such factor is a build-up of error due to tolerances in the manufacturing of the mold plate in which the gates are positioned, the manifold plate to which the nozzles are attached, and the nozzles themselves. Another factor is that the mold plate and the manifold plate may undergo different rates of thermal expansion during the injection molding process, and the nozzles may be moved out of alignment with respect to the gates as a result.
The adjustment of the positioning of the nozzles on an injection molding apparatus is typically carried out on each nozzle individually, by re-machining each of the nozzle components until a satisfactory alignment is obtained. This can be a time-consuming process.
Furthermore, other problems exist with the alignment of the valve pin. For example, during use, melt typically surrounds the valve pin, and also surrounds the downstream end of the nozzle itself in some injection molding machines. If the melt does not exert pressure evenly on the valve pin or on the nozzle, the valve pin and/or the nozzle can be moved out of alignment because of the melt pressure.
Some nozzles do not incorporate a valve pin. For example, some nozzles include a ‘torpedo’ which is a tip that is generally conical at its downstream end and extends into the gate, to control the heating of the melt as close as possible to the entry of the melt into the mold cavity.
If the torpedo is even slightly misaligned, the flow into the mold cavity can become non-uniform and can cause several problems with the quality of the molded part. Injection molding apparatuses having nozzles with torpedoes suffer from similar problems to valve gated machines, in terms of misalignment and consequent re-machining of components.
A second issue exists with hot runner nozzles, and that is the need to control the temperature of melt in the nozzle. Temperature control of the melt is even more important towards the outlet of the nozzle.